This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-313854, filed Oct. 13, 2000; and No. 2001-273826, filed Sep. 10, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a cathode-ray tube (CRT) apparatus, and more particularly to a color cathode-ray tube apparatus capable of reducing a difference between a focusing power of a lens function acting on a center beam and a focusing power of a lens function acting on side beams, thus obtaining a uniform resolution over the entire area of a screen.
2. Description of the Related Art
An in-line self-convergence type color CRT apparatus includes an in-line type electron gun assembly for emitting three in-line electron beams. The performance of a main lens of the electron gun assembly is expressed by lens constants, e.g. a lens magnification and a spherical aberration coefficient. In particular, these two constants substantially determine the performance of the lens.
The less the lens constants, the better the lens performance. As these lens constants decrease, the electron beams can be focused with smaller beam spots on the screen. Accordingly, a higher resolution can be obtained.
One means, which has been proposed to enhance the lens performance, is an electron gun assembly having an electric field expansion type main lens that increases a main lens region in a tube axis direction and virtually enlarges the diameter of the main lens. This electric field expansion type main lens is composed by increasing a distance (lens gap) between the electrodes of the main lens and disposing at least one intermediate electrode between the electrodes.
The lens performance of the electron gun assembly is not sufficient to provide the color CRT apparatus with a good resolution over the entire area of the screen. Specifically, the electron beams emitted from the electron gun assembly are deflected over the entire screen by deflection magnetic fields produced by a deflection yoke. The deflection magnetic fields, however, have field distribution configurations distorted, e.g. in a barrel shape or a pincushion shape, in order to substantially converge the three electron beams at one point over the entire screen.
The distortion in the deflection magnetic fields disadvantageously deforms the beam spot shapes of the electron beams that have landed on the phosphor screen. A beam spot on a peripheral portion of the screen comprises a high-luminance core portion, which is horizontally elongated due to under-focusing, and a low-luminance blur portion, which has been vertically elongated due to over-focusing. This degrades the resolution of the screen.
A so-called dynamic focus type electron gun assembly has been proposed as a means for solving the problem of the deformation of the beam spot due to the distortion of deflection magnetic fields. The electron gun assembly includes an asymmetric lens having a lens power varying in synchronism with the deflection of the electron beam, thereby to cancel the deformation of the beam spot due to the deflection magnetic fields.
The asymmetric lens has a lens function of focusing, with a weak vertical focusing power, the electron beams to be focused on a peripheral portion of the screen. In addition, the beam spot on the peripheral portion of the screen is horizontally focused in an almost optimal state. Thus, the asymmetric lens is designed to maintain a fixed focusing power in an optimal state by canceling the variation of the lens power of the main lens in the horizontal direction.
Side beam passage holes in the respective electrodes of the main lens are made eccentric in order to statically converge the three electron beams at substantially one point on the screen. As a result, the trajectories of the side beams in the main lens are inclined to the center axis of the electron gun assembly.
Accordingly, the side beams passing through the side beam passage holes are affected by the lens function of the main lens over a longer distance than the center beam. Thus, the side beams are over-focused, compared to the center beam. Consequently, a blur occurs on the screen, and this degrades the resolution.
In particular, at the peripheral portion of the screen, the image point distance from the main lens to the phosphor screen is longer than at the central portion of the screen. This increases the difference between the focusing power of the lens function acting on the center beam and the focusing power of the lens function acting on the side beams. The side beams reaching the peripheral portion of the screen are greatly over-focused, and blurred beam spots are created. This greatly degrades the resolution at the peripheral portion of the screen. This tendency becomes more conspicuous in an electron gun assembly wherein plural intermediate electrodes are used and the main lens region is greatly increased. In other words, as the diameter of the electric field expansion type main lens is increased to enhance the lens performance, the resolution will deteriorate more considerably.
The present invention has been made in consideration of the above problems, and the object of the invention is to provide a cathode-ray tube apparatus capable of reducing a difference between a focusing power of a lens function acting on a center beam and a focusing power of a lens function acting on side beams, thus obtaining a uniform and high resolution over the entire area of a screen.
In order to solve the problems and achieve the object, the invention of claim 1 provides a cathode-ray tube apparatus including:
an electron gun assembly having an electron beam generating section which generates three electron beams consisting of a center beam and a pair of side beams positioned on both sides of the center beam, and a main lens section which focus the electron beams generated by the electron beam generating section onto a phosphor screen; and
a deflection yoke which deflects the electron beams emitted from the electron gun assembly in a horizontal direction and a vertical direction,
wherein the main lens section comprises a focus electrode, to which a focus voltage of a first level is applied, an anode, to which an anode voltage of a second level higher than the first level is applied, and at least one intermediate electrode which is disposed between the focus electrode and the anode and to which a voltage of a substantially intermediate level between the first level and the second level is applied,
the focus electrode has, in a first end face thereof opposed to the intermediate electrode, three electron beam passage holes for passing the three electron beams,
the electron gun assembly includes an asymmetric lens section which has such asymmetry that lens functions of the asymmetric lens section acting on the electron beams are different between the horizontal direction and the vertical direction, and which has a lens power varying in synchronism with the deflection of the electron beams, and
the asymmetric lens section is created at a position away by a distance or less, which corresponds to a diameter of the electron beam passage hole formed in the first end face, from the first end face of the focus electrode toward the electron beam generating section, the asymmetric lens section having an asymmetry of a lens function acting on the center beam, which differs from an asymmetry of a lens function acting on the side beams.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.